The present invention is directed, in general, to a method of manufacturing a semiconductor device and, more specifically, to a method of creating a hydrogen isotope reservoir in the semiconductor device at a relatively higher processing temperature to maximize retention of the hydrogen isotope during subsequent processing steps. This allows device to perform without any drift in transistor characteristics under hot-carrier aging (HCA).
The use of silicon in semiconductor devices, such as metal-oxide-semiconductor-field-effect-transistor(xe2x80x9cMOSFETxe2x80x9d), well known. Equally well known is the degradation of these devices, under hot carrier stressing. A result of hot electron emission into the gate oxide is electron trapping with the oxide. Typical silicon dioxide films trap only about one percent of the total number of injected electrons. Electron trapping, of course, results in a negative charge accumulation within the gate oxide and thus will eventually change the MOSFET behavior. Specifically, the addition of negative charge raises the MOSFET threshold voltage since more positive gate bias is required to overcome the negative oxide charge. Increased threshold voltage is detrimental since the MOSFET current drive decreases. The degradation in MOSFET performance is minimal in the saturation regime and much more pronounced either in the linear regime or with source and drain terminals reversed so that the hot electron damage is localized at the source end of the channel.
Quite intriguing is the well-known result that hot electron degradation consists also of interface states. Interface states are electron energy states within the forbidden silicon band gap at the silicon-silicon dioxide interface. These states are detrimental since they can temporarily trap charge and thus change MOSFET characteristics or reduce charge carrier mobility within the channel. Charge pumping and low-frequency noise measurements have confirmed the localized nature of these surface states. In addition to increased threshold voltage, another result of hot electron degradation is reduced linear region transconductance. In fact, it is more common to report transconductance data since the parameter tends to degrade before one observes any changes in threshold voltage. Transconductance degradation is often loosely explained by channel mobility reduction due to surface state generation, but modeling studies have shown that localized electron trapping can also explain the loss in transconductance. Channel hot electron degradation increases in severity as the n-channel MOSFET channel length decreases. As we noted previously, hot electron injection is a strong (exponential) function of the mean electron energy, and this mean energy increases with increasing electric field. One specifies reduced channel length, gate oxide thickness, and source-drain junction depth, as well as increased channel acceptor impurity concentration. All of these modifications act to increase the maximum electric field in the channel and thus exacerbate the hot electron problem.
With respect to interface traps, it is believed that the interface traps are caused by defects that are generated by current flow in such semiconductor devices. It is further believed that these defect states reduce the mobility and lifetime of the carriers and cause degradation of the device""s performance. In most cases, the substrate comprises silicon, and the defects are thought to be caused by dangling bonds (i.e., unsaturated silicon bonds) that introduce states in the energy gap, which remove charge carriers or add unwanted charge carriers in the device, depending in part on the applied bias. To alleviate the problems caused by such dangling bonds, a passivation process of hydrogen isotopes has been adopted and established in the fabrication of such devices.
In the hydrogen/deuterium passivation process, it is thought that the defects that affect the operation of semiconductor devices are removed when the deuterium bonds with the silicon at the dangling bond sites. In order to impregnate the semiconductor devices with hydrogen and deuterium, at a higher concentration, a high temperature process such as gate oxidation or poly deposition is adopted where hydrogen isotope is introduced during oxidation or poly deposition at energetically favorable sites such as interfaces within the oxide or within the poly gate electrode layers. Frequently, the hydrogen or deuterium that is pumped into the semiconductor device diffuses out during subsequent thermal processing. Thus, while the hydrogen/deuterium passivation process eliminates the immediate problem associated with dangling silicon bonds, it does not eliminate degradation permanently because the hydrogen/deuterium atoms that are added by the passivation process can be xe2x80x9cdesorbedxe2x80x9d or removed from the previous dangling bond sites by the next thermal process or under hot-carrier aging.
A hot carrier is an electron that has a high kinetic energy, which is imparted to it when voltages are applied to electrodes of the device. Under such operating conditions, the hydrogen/deuterium atoms, which were added by the passivation process, are knocked off by the hot electrons. This hydrogen/deuterium desorption results in aging or degradation of the device""s performance. According to established theory, this aging process occurs as a result of hot carriers stimulating the desorption of the hydrogen from the silicon substrate""s surface or the silicon dioxide interface. This hot carrier effect is particularly of concern with respect to smaller devices in which proportionally larger electric fields can be used.
Accordingly, what is needed in the art is a semiconductor device and a method of manufacture therefore that addresses the deficiencies of the prior art.
To address the above-discussed deficiencies of the prior art, the present invention provides a method of manufacturing a semiconductor device. In an advantageous embodiment, the method includes creating a hydrogen isotope sink within a semiconductor material located on a semiconductor substrate by oscillating a deposition parameter during a formation of the semiconductor material and incorporating a hydrogen isotope, such as deuterium, into the semiconductor material at the interface created by the oscillation. The hydrogen may be incorporated either during or after the step of creating a hydrogen isotope sink.